Grow lights are good only when they are at the proper distance. But often, we end up either burning our plant with too much light or leaving the plant with insufficient light to grow. Can we scientifically compute how far should grow lights be from plants?

Natural sunlight gently filters through the atmosphere, and the plants can enjoy a lovely, softer light even on bright days. But most grow lights shine a more intense beam of photosynthetically active radiation (PAR).

If these lights are positioned too closely, they can easily scorch leaves or stress the plants. So, finding that perfect distance—the “sweet spot”—is essential for ensuring plants receive just the right amount of light for healthy growth without any harm.

This post aims to help you find the answer to this question.

How do you know if grow lights Are positioned in the correct distance?

Plants need light to grow, but they need light in different amounts. The amount is the plant’s daily light integral (DLI), measured in mols/m²/day. Plants can survive a day or two without meeting this light requirement. But if they consistently miss this level, they might show some undesirable signs.

For instance, lettuce grows well at around 11.5 mols/m²/day. However, one study shows that lettuce grown at 8.64 and 14.4 mols/m²/day has significantly low fresh weight.

But before getting to the harvesting stage, we can find the signs and fix them quickly. Here are some signs you need to be watchful.

Signs of Insufficient Light

Plants that aren’t receiving enough light may exhibit:

• Leggy Growth: Stretched, elongated stems as plants search for light.
• Pale or Yellow Leaves: Reduced pigmentation (chlorosis) due to low photosynthesis.
• Small or Sparse Foliage: Fewer and smaller leaves.
• Slow or Stunted Growth: Overall reduced vigor and delayed development.

Signs of Too Much Light Intensity

When the grow light is too close or too intense, you might see:

• Leaf Burn or Scorching: Brown or crispy edges on leaves.
• Bleaching or Discoloration: Leaves may turn white or yellow where light damage has occurred.
• Leaf Curling: A protective response to excessive light and heat.
• Wilting or Stress Symptoms: Despite adequate watering, plants may appear stressed.

These signs act as a compass for proper growth and light distancing. When you see light stress, you can elevate the growth light higher, and when you see signs of stagnant growth, you move them closer.

This is the most practical and easiest way of determining how far should grow lights be from plants. But we can do it in a small setup, not a large-scale farm.

Active monitoring is often practiced in industrial farms, but it’s costly. Nonetheless, chances for error must be kept low to avoid catastrophic results.

So let’s get into the technicals.

How light is measured in horticulture: PPE, PPF, PPFD, & DLI.

Although we can’t touch or feel light, we can measure it. However, people often confuse the two different measurements: Lux and PPFD.

Lux measures the amount of light needed for human needs. Generally, residential and commercial lighting are designed to emit more yellow and blue than red, which makes the light cool and bright.

However, red is crucial for plant growth. Flowering plants need an even bigger dose of red.

Related: Grow Light vs Regular Light for Indoor Plants

Ppfd measures light intensity in the spectral range that is suitable for plant growth. By definition, PPFD represents the number of photons (µmol) withing 400–700nm wavelength striking a square meter area (m²) every second (s). The 400–700 nanometers (nm) wavelength is often reffered to as the Photosynthetically Active Radiation (PAR).

PPFD of sunlight usually peaks at around 1800-200 µmol/m²/s near the equator. But they get significanlty low as we move towards teh polars.

Related: Comparing Grow Light vs Sunlight for Optimal Growth.

Grow lights produces sufficient PPFD for plant growth. But as grow lights are moved away from the plant, they go down and they go up as we move closer. Also, PPFD values are usually very high at the center of the growign area, and low near the edges.

You can find this information on the technical specification published by the grow light manufacturers. It’s called the PAR map.

PPFD is essentially the density of photones. But high PPFD for a short duration isn’t very helpful to plants. Plants should enjoy some decent PPFD for sufficient duration to absorb enough of light. This is the daily light integral (DLI).

For instance, PPFD of 400 for a 12 hour photoperiod means a DLI of 17.28mol/m² /day.

We now know what PPFD and DLI means. We will use this in the next section to compute the grow ligth coverage. But before that we need to get a few more technical terms clarified.

PPE, PPF, and Usable PPF

Grow lights convert electricity into PAR light. Modern grow lights are very efficient. But no machine converts 100% energy into a desirable outcome. Grow lights don’t, either.

The amount of light produced (moles) for every joule of energy the grow light uses is called PPE (Photosynthetic Photon Efficacy).

The regular lights, such as the T5 fluorescent light, often have a PPE of less than 2 µmol/j. This is highly inefficient for growing plants. But Modern LED grow lights go even close to 4 µmol/j. This means that grow lights produce around twice as much PAR as regular lights.

PPF is the total amount of photons produced by the light every second. A 100W LED grow light uses 100 joules of energy every second. If its PPF is 3.5 µmol/j, it can produce 100 j X 3.5 µmol/j = 350 3.5 µmol every second. Therefore, 350 µmol/s is the PPF of this grow light.

However, not all these photos are productive in real life. About 15% of this doesn’t reach the plant canopy. If we account for this, 350 µmol/s X 0.85 = 297.5 µmol/s is the usable PPF of this grow light.

Computing the Coverage Area for Optimal Plant Growth

We’ve got the basics of all the essential grow light specs. Now, it’s time to put them all together.

We are trying to link a grow light’s PPF to a plant’s DLI. Let’s return to lettuce.

Lettuces have an optimal DLI of 11.5 mol/m²/day, corresponding to a PPFD value of 266 µmol/m²/s. If a grow light’s usable PPF is 524 µmol/s, we can cover up to 2 m² of space.

Here’s the step-by-step calculation:

If you find this computation complex, or don’t want to do it every time, we’ve got a free tool for this. Check out our grow light coverage calculator.

Understanding the coverage is key to estimate how high the grow lights should be.

You could move the grow light up and down to get the desired coverage. But this approach is not very convenient.

Instead we can compute the distance based on the light angle provided by the manufacturer. For most grow lights, this is around 120 degrees, although some can go upto 160 degrees. But since 120 is something we can rely on, we always assume this.

Here’s the computation to find the grow light distance.

That is roughly 18 inches above the canopy.

Of course, this method isn’t perfect all the time. There are many other factors contribute to the grow light distance. For instance, the PPFD near the edge of the grow area is significantly lower than the center. This computation doesn’t take that into account. If you use reflectors, it’ll alter the computation. That is another factor not taken into account. Also, in industrial setup, grow lights are placed one after the other, the light from another grow light is still a factor to consider.

Nonetheless, having a theoretical idea of how far should grow lights be from plants is still helpful. For an adjusted list of distances, please consult the table in the next section.

Plants, Their Growing Stages, and Suggested Grow Light Distance from the Canopy

Below is a sample table with typical recommendations. (Note: Actual distances may vary depending on the specific light type, wattage, and environmental conditions.)

*For basil, “flowering” is less emphasized since it’s grown primarily for leaves.

Why Different Plants & Growing Stages Need Different DLI

Plants have evolved to thrive under different light conditions. For instance, coconuts grow well in tropical lands, lettuces grow in cooler and less intense light conditions. That’s all normal because they’ve originated in different parts of the world and evolved facing differnt challenges.

We think the three main categories of plant you should know when growing under a grow light are shade plants, leafy greens & herbs, and flowring/fruiting plants. Here are some general assumption of light requirement for these plant varieties.

• Shade Plants (e.g., many tropical foliage species, ferns):
  Adapted to low-light environments; they require a lower DLI (often 4–10 mol/m²/day) to avoid photodamage.
• Leafy Greens & Herbs (e.g., lettuce, spinach, basil):
  Require moderate light levels, typically between 10–17 mol/m²/day, to produce compact, high-quality foliage.
• Flowering/Fruiting Plants (e.g., tomatoes, peppers, cucumbers):
  Need higher DLIs (20–30 mol/m²/day or more) to promote flowering, fruit set, and higher yields.

Related: Low-Light Hydroponic Plants You Are Most Likely to Succeed in Growing

The variation is due to differences in leaf anatomy, photosynthetic capacity, and overall plant physiology.

In a very similar way, plants’ lighting needs change according to their growth stages too. Because plants are more sensitive at certain stages, the recommended light distance changes:

• Germination:
  Requirement: Very gentle light.
  Recommended Height: Place lights further away (e.g., 24–36 inches) to prevent burn and reduce heat exposure.
• Seedling Stage:
  Requirement: Delicate plants need moderate intensity.
  Recommended Height: Approximately 18–24 inches above the seedlings.
• Vegetative Stage:
  Requirement: Rapid growth and higher photosynthesis.
  Recommended Height: Lower the light closer (e.g., 12–18 inches) to increase PPFD without causing stress.
• Flowering Stage:
  Requirement: High-intensity light to maximize bud development.
  Recommended Height: Often similar to or slightly lower than the vegetative stage (around 12–18 inches), but always monitor for signs of light burn.

Grow light distancing in industrial farms

Agronomists use the theoretical approach to determine the optimal grow light distance, but this alone is insufficient.

Also, as we said earlier, if you grow plants for a living, you can’t afford to wait until the plants show signs of light issues.

The best approach is to use a PAR meter. The PAR meter can tell you precisely what the PPFD is where the sensor is. If you know the plant DLI and control the photoperiod, you can find the required PPFD. Then, you can adjust the grow light height to match the plant’s needs.

This is often the practice in industrial setups. But it’s often a one-time study. For subsequent batches, you don’t have to do it repeatedly if you’re using artificial lighting alone.

For hobbyists and small-scale growers, a light meter can be too expensive. Try the Photone appwhich is mostly accurate and has comparable results to an expensive gadget.

Final Thoughts

Plant’s need light. But just the right amount. Under sunlight, you have little control over the light intensity and other properties. But you can take advantage of grow light because you can distance them as you wish.

Proper grow light distiancing isn’t easy. But with the knowledge of this post, you can compute the hight. Though this height isn’t always the best, it’s a good starting point.

You can now adjust the light to get the sweet spot.